Phenol is an important product in the chemical industry. For example, phenol is useful in the production of phenolic resins, bisphenol A, ε-caprolactam, adipic acid, alkyl phenols, and plasticizers.
Currently, the most common route for the production of phenol is the Hock process. This is a three-step process in which the first step involves alkylation of benzene with propylene to produce cumene, an alkylaromatic compound in the presence of an acidic catalyst. The second step is the oxidation, preferably aerobic oxidation of cumene to the corresponding cumene hydroperoxide. The third step is the cleavage of the cumene hydroperoxide in the presence of heterogeneous or homogenous catalysts into equimolar amounts of phenol and acetone, a co-product. However, the world demand for phenol is growing more rapidly than that for the acetone co-product. In addition, due to developing shortages in supply, the cost of propylene is likely to increase.
Thus, a process that avoids or reduces the use propylene as a feed and coproduces higher ketones, such as methyl ethyl ketone and/or cyclohexanone, rather than acetone may be an attractive alternative route to the production of phenol. For example, methyl ethyl ketone is in demand for use as a lacquer and a solvent and for dewaxing of lubricating oils. In addition, there is a growing market for cyclohexanone, which is used as an industrial solvent, as an activator in oxidation reactions and in the production of adipic acid, cyclohexanone resins, cyclohexanone oxime, caprolactam and nylon 6.
It is known that phenol and methyl ethyl ketone can be co-produced by a variation of the Hock process in which sec-butylbenzene is oxidized to obtain sec-butylbenzene hydroperoxide and the hydroperoxide is decomposed to the desired phenol and methyl ethyl ketone. The sec-butylbenzene can be produced by alkylation of benzene with linear butenes over zeolite beta or a molecular sieve of the MCM-22 family. Details of such a process can be found in, for example, International Patent Publication No. WO2006/015826.
Similarly, U.S. Pat. No. 6,037,513 discloses that cyclohexylbenzene can be produced by contacting benzene with hydrogen in the presence of a bifunctional catalyst comprising a molecular sieve of the MCM-22 family and at least one hydrogenation metal selected from palladium, ruthenium, nickel, cobalt and mixtures thereof. The '513 patent also discloses that the resultant cyclohexylbenzene can be oxidized to the corresponding hydroperoxide which is then decomposed to the desired phenol and cyclohexanone co-product.
In the commercial Hock process, the dilute cumene hydroperoxide from the oxidation step is first concentrated to greater than 80% under vacuum, which is then sent to the cleavage reactor. In addition to the hazards associated with handling concentrated hydroperoxide, cleavage of concentrated hydroperoxide also poses safety concerns due to the rapid and highly exothermic nature of the reaction. Further, significant amounts of side products may also form from the concentrated oxidation products. In practice, the concentrated cumene hydroperoxide is often diluted with solvents such as acetone in order to better manage the reaction heat and to control by-product formation.
In the case of cyclohexylbenzene oxidation, another disadvantage of concentrating cyclohexylbenzene hydroperoxide exists. Due to the very high boiling point of cyclohexylbenzene, high vacuum and high temperature are required in order to remove cyclohexylbenzene and concentrate cyclohexylbenzene hydroperoxide, which may lead to undesired decomposition of cyclohexylbenzene hydroperoxide.
In addition, the production of phenol using sec-butylbenzene and/or cyclohexylbenzene as the alkylbenzene precursor is accompanied by certain problems which either is not present or is less severe with a cumene-based process. For example, in comparison to cumene, oxidation of sec-butylbenzene and cyclohexylbenzene to the corresponding hydroperoxide is very slow in the absence of a catalyst and is very sensitive to the presence of impurities. As a result, U.S. Pat. Nos. 6,720,462 and 6,852,893 have proposed the use of cyclic imides, such as N-hydroxyphthalimide, as catalysts to facilitate the oxidation of alkylbenzenes, such as sec-butylbenzene and cyclohexylbenzene.
All of these problems increase the complexity and investment involved in the cleavage process and hence various alternatives have been proposed.
The synthesis of phenol and cyclohexanone in one pot with selectivities of 96% and 91%, respectively, at 25% conversion, by means of aerobic oxidation of cyclohexylbenzene in the presence of the N-hydroxyphthalimide, followed by treatment with sulfuric acid, as disclosed by Aoki et al. in Tetrahedron, Vol. 61 pages 5219-5222 (2005).
According to the present invention, it has now been found that cleavage of hydroperoxides directly out of the oxidization reactor may be achieved without the need of first concentrating the hydroperoxides. The un-reacted alkyl benzenes act as diluents for the peroxides, which can help manage the heat of cleavage reaction and better control formation of heavy by-products. Phenol and co-products are produced with selectivities of greater than or equal to 98%.